Composites

ABSTRACT

A method of fabricating a composite material, the method comprises the steps of a) providing a first layer of a fibre reinforced polymer, preferably a thermoset FRP, b) providing an array of thermoplastic islands across at least a proportion of a major surface of the first layer, c) providing a second layer of a fibre reinforced polymer, preferably a thermoset FRP, d) laying the second layer over at least some of the islands, and e) securing the first and second layers together. There is also disclosed a composite which comprises a first layer of a fibre reinforced polymer and a second layer of a fibre reinforced polymer, between which is an intervening layer comprising an array of thermoplastic islands.

This invention relates to a composite and a method of producing acomposite. In particular, the invention relates to composites which showbeneficial properties.

In particular, although not exclusively, this invention relates to atoughened, preferably self-healing composite and a method of producing atoughened self-healing composite. Advantageously, although notexclusively, the invention relates to composites which show beneficialproperties without introducing additional weight (more than 1%) bylaying down, e.g. printing, additives between composite plies

Composite materials are becoming ever more valuable and are being usedin increasing volumes. For example, as well as the well-known use inBoeing's 787 and Airbus' A350 aircraft, composites are increasingly usedfor structural projects, such as railway bridges and so on. As thegeneral acceptance of composite materials increases the uses to whichthey are put, and the concomitant demands thereon will likely increase.

One of the major issues with composite materials is the ability todetect damage in situ. Another is repair of composite materials partsonce damage has been detected. There is clearly a desire to havestronger (for example stiffer and/or more resistant to interlaminarshear forces) materials. There is a further desire to have stronger(e.g. stiffer and/or more resistant to interlaminar shear forces)materials which are not substantially heavier than those they replace.

It is an object of the current invention to provide anew compositematerial which is able to satisfy one or more of the above criteria.

A first aspect of the invention provides a composite material comprisinga first and second layer and an intervening layer, the first and secondlayer comprises a fibre reinforced polymer, the intervening layercomprises an array of islands of thermoplastic polymer.

Preferably, the fibre reinforced polymer (FRP) is a thermoset FRP.

In a further aspect of the invention, there is provided a compositematerial comprising a first and second layer and a thermoplasticmaterial, the first and second layer comprises a thermoset fibrereinforced polymer, the thermoplastic material is discontinuouslydistributed at and/or towards the interface between the first and secondlayer.

Preferably, the discontinuous distribution is in the form of an array ofislands or zones, for example islands or zones of peak localconcentration.

The discontinuous distribution may extend over some or all of or towardsthe interface, and/or may be localised in certain defined regions.Additionally or alternatively, the density of thermoplastic material mayalter across the substrate.

One or more or all of the islands, or each island, may comprise morethan one thermoplastic polymer and/or may comprise a copolymer.

The first and second layer may be formed from the same or differentthermoset polymers and/or may comprise the same or different fibres.

The thermoset polymers may comprise an epoxy, polyesters, vinyl esters,phenolic resins and any other cross-linked thermosetting polymer.

The fibres may be selected from one or more of carbon fibres, aramidfibres, glass and glass ceramic fibres, plastics fibres (e.g. Nylon,Terylene), hemp, wood and/or other organic fibres, inorganic fibres. Thefibres may be provided as continuous and/or discontinuous fibres. Thefibres may comprise aligned filaments or tows, or as a braided, knittedor woven mat or sheet. Continuous fibres may be provided as athree-dimensional or two-dimensional structure within the first and/orsecond layer.

Preferably, the volume fraction of fibre-to-polymer is from 0.01 to 5%,most preferably from 0.05 to 1%

In a preferred embodiment the first and second layer comprises a carbonfibre reinforced thermoset polymer, preferably an epoxy polymer.

The islands may comprise one or more thermoplastic polymers. Forexample, a first array of islands and a second array of islands, each ofthe first and second arrays being formed of different thermoplasticsmaterials. Further arrays may be provided. The arrays may be regular orirregular.

The thermoplastic polymer may have a melting point of 50 to 200° C.,preferably 75 to 190° C., most preferably 100 to 180° C. The molecularweight of the thermoplastic polymer is preferably in the range of 3000to 50000, preferably 4000 to 40000 (measured as the number averagemolecular weight). The thermoplastic material may be applied as asolution of thermoplastic material in a solvent. The thermoplasticmaterial may be provided as a solution of up to 50 wt/wt % thermoplasticmaterial, for example up to 45, 40, 35, 30, 25 wt/wt % and preferablymore than 1 wt/wt %, for example from 1 to 25 wt/wt %, and mostpreferably from 4 to 21 wt/wt %.

The thermoplastic polymer may be selected from poly(ethylene glycol)(PEG), poly(methyl methacrylate) (PMMA) and similar thermoplasticpolymers, preferably poly(ethylene glycol) and poly(methylmethacrylate).

The islands preferably extend over 20-80% of the area of the firstlayer, preferably 40-60%.

Preferably each of the islands each have an area of less than 1 mm²,preferably less than 0.5 mm², for example, less than 0.4, 0.3, 0.2, 0.1mm².

Most preferably, the islands are formed in a regular array, for examplein columns and rows, or an array aligned along the nominal vertices ofpolyhedra, for example hexagons, pentagons or other tessellating shapes.

The composite may comprise a further intervening layer comprising anarray of islands of thermoplastic polymer provided on the second layer.There may be provided a third layer comprising a fibre reinforcedpolymer, preferably a thermoset FRP, overlying the further interveninglayer. There may be one or more successive intervening layers and fibrereinforced polymer layers.

A further aspect of the invention provides a method of fabricating acomposite material, the method comprising the steps of

-   -   a) providing a first layer of a fibre reinforced polymer,        preferably a thermoset FRP,    -   b) providing an array of thermoplastic islands across at least a        proportion of a major surface of the first layer,    -   c) providing a second layer of a fibre reinforced polymer,        preferably a thermoset FRP,    -   d) laying the second layer over at least some of the islands,        and    -   e) securing the first and second layers together.

The first and second layers may be secured together by the applicationof heat and/or pressure.

The fibres may be may be selected from one or more of carbon fibres,aramid fibres, glass fibres, plastics fibres (e.g. Nylon, Terylene),hemp, wood and/or other organic fibres, inorganic fibres. The fibres maybe provided as continuous and/or discontinuous fibres. The fibres maycomprise aligned filaments or tows, or as a braided, knitted or wovenmat or sheet. Continuous fibres may be provided as a three-dimensionalor two-dimensional structure within the first and/or second layer.

The thermoset polymers may comprise an epoxy, polyesters, vinyl esters,phenolic resins and the like.

Heat may be used to secure the two layers together.

Preferably, the first and second layers comprise a prepreg, for examplea carbon fibre reinforced prepreg, preferably an epoxy matrix. In whichcase, the application of heat will continue the curing process of theprepreg polymer and will cause the first and second layer to becomesecured together.

Other methods of securing the two layers are also envisaged, includingresin injection (e.g. RTM and/or VARTM), adhesives and so on.

The thermoplastic islands may be applied as a solution of thermoplasticin a solvent. The thermoplastic may be present at 1 to 50 w/w % ofsolution, may from 1 to 45 or 40 w/w %, most preferably from 2 to 40 w/w%, say 3 to 30 w/w %.

The solvent may be non-polar or polar. Polar solvents may be protic oraprotic. The solvent will, in part, depend upon the thermoplastic to bedissolved. Suitable solvents may include water, C₂-C₈ primary, secondaryor tertiary alcohols (for example ethanol), N, N-Dimethylformamide.

The thermoplastic polymer may have a melting point of 50 to 200° C.,preferably 75 to 190° C., most preferably 100 to 180° C. The molecularweight of the thermoplastic polymer is preferably in the range of 3,000to 50,000, preferably 4,000 to 40,000 (measured as the number averagemolecular weight, M_(n)).

The thermoplastic polymer may be selected from poly(ethylene glycol),poly(methyl methacrylate) and like polymers, preferably poly(ethyleneglycol) and poly(methyl methacrylate).

The islands preferably extend over 20-80% of the area of the firstlayer, ideally 40-60%.

Preferably each of the islands will be laid down with an area of lessthan 1 mm², preferably less than 0.5 mm², for example, less than 0.4,0.3, 0.2, 0.1 mm².

The islands may be laid down by an application method, for example usingink jet printing, contact printing and non-contact printing or by use ofa mask and spray method. Alternatively, the islands may be laid down asa continuous layer and parts thereof removed (by physical or chemicalmeans) to provide the required array. Physical means may includeabrasion, application of heat and so on and chemical means may includedissolution. For ease of manufacture, application methods are preferred,with ink jet printing being particularly preferred.

Clearly the application of heat will cause the solvent to evaporate.However, we believe that evaporation of the solvent will not cause asubstantial reduction in the size of the islands (and hence areacovered), but rather a volume reduction in the island.

The islands may comprise one or more thermoplastic polymers. For examplea first array of islands and a second array of islands, each of thefirst and second arrays being formed of different thermoplasticsmaterials. Further arrays may be provided. The arrays may be regular orirregular.

Most preferably, the islands are formed in a regular array, for examplein columns and rows, or an array aligned along the nominal vertices ofpolyhedra, for example hexagons, pentagons or other tessellating shapes.

The method may further comprise repeating steps b) to e) to formsuccessive layers of the composite material.

Whilst we do not intend to be bound by any particular theory, wepostulate that the provision of the intervening thermoplastic layer(s)improves the self-healing properties of the composite material, e.g.,after heat application. Further, we believe that providing a patternedarray of thermoplastic material allows the preceding and succeedinglayer to form intimate bonds which reduces the likelihood ofdelamination due to shear forces. The balance of the provision ofsufficient thermoplastic to encourage or allow self-healing whilstensuring the physical coherence of the structure is particularlyattractive. Moreover, we believe that the deployment of direct lay downmethods (e.g. ink jet printing) is particularly advantageous in terms ofmanufacturing flexibility and scale up. Finally, this method enablesadvantageous increase in disparate mechanical properties such asstiffness and toughness of the system, without introducing an additionalweight, rendering it very desirable for transport systems and anyproducts that use power for motion.

A further aspect of the invention provides a composite, preferable aself-healing composite, the composite comprising a first layer of fibrereinforced thermoset polymer and a second layer of fibre reinforcedthermoset polymer and a thermoplastic polymer distributed at and/ortowards the interface there between, the composite being stiffer than anequivalent fibre reinforced thermoset polymer composite absent thethermoplastic polymer.

A yet further aspect of the invention provides a composite, preferable aself-healing composite, the composite comprising a first layer of fibrereinforced thermoset polymer and a second layer of fibre reinforcedthermoset polymer and a thermoplastic polymer distributed at and/ortowards the interface there between, the composite having improvedstiffness and/or interlaminar shear strength.

The thermoplastic polymer may be discontinuously distributed at and/ortowards the interface between the first and second layer. Thethermoplastic polymer may be preferentially distributed at or towardsthe interface such that relative concentration of thermoplastic polymeris at a peak at or towards the interface.

Preferably, the discontinuous distribution is in the form of an array ofislands or zones, for example islands or zones of peak localconcentration.

In this specification, the term “improved stiffness and/or interlaminarshear strength” means that the composite material has an increase in oneor other (or both) of the property of stiffness or interlaminar shearstrength, when compared with an equivalent structure which is absent theappropriately distributed thermoplastic polymer.

The composite will be substantially the same weight as a correspondingcomposite absent the thermoplastic polymer.

In order that the invention may be more fully understood, it shall nowbe described, by way of example only, with reference to the accompanyingexamples and drawings, in which:

FIG. 1 is a series of optical microscopy slides related to Example 1A;

FIG. 2 is a series of optical microscopy slides related to Example 1B;

FIG. 3 is a series of optical microscopy slides related to Example 1C;

FIG. 4 is a series of optical microscopy slides related to Example 1D;

FIG. 5 is a series of optical microscopy slides related to Example 1E;

FIG. 6 is a series of optical microscopy slides related to Example 1F;

FIG. 7 is a series of optical microscopy slides related to Example 2A;

FIG. 8 is a series of graphs related to Example 3

FIG. 9 is a series of graphs related to Example 4;

FIG. 10 is a series of graphs related to Example 5;

FIG. 11 is a series of graphs related to Example 6;

FIG. 12 is a graph related to Comparative Example 7;

FIG. 13 is a graph related to Example 8;

FIG. 14 is a graph related to Example 9;

FIG. 15 is a graph related to Example 10;

FIG. 16 is a series of graphs related to further results

FIG. 17 is a schematic representation of a part made according to theinvention;

FIG. 18 is a graph associated with testing the part of FIG. 17;

FIG. 19 is a graph showing the effect of increasing polymerconcentration;

FIGS. 20 and 21 are a series of graphs showing the stiffness of virginand test composites before and after heating respectively.

In a first set of experiments we investigated the application of variousthermoplastic polymers onto a carbon fibre epoxy prepreg.

EXAMPLES 1 TO 3

In a first set of experiments we dissolved thermoplastics materials, invarious solvents, in the amounts set out in Table 1 below:

TABLE 1 Parameters of thermoplastic solution and print characteristicsComposition of ‘ink’ Diameter of Example Solute wt % Solventprinthead/μm Pattern 1 PMMA 5 DMF* 60 Hexagon 2 PEG 5 Distilled 60Hexagon water 3 PEG 5 Pure Ethanol 60 Hexagon Where: Mn_((PMMA)) =15,000; Mn_((PEG)) = 20,000; DMF is N,N-Dimethylformamide; Pattern isthe shape described by the array of dots.

The ink jet printer used was a Jetlab 4XL (supplied by MicroFab) andoperated in each case according to the parameters as set out in Table 2.

TABLE 2 Parameters for operation of ink jet printer Example Jet 1 RT 2RT 3 Polymer Where: RT is room temperature and Polymer is Polymer JetPrintHead, which can be heated to temperatures as high as 240° C.

In each case the Rise Time, Dwell Time and Dwell Voltage may be alteredto ensure that stable droplets are generated.

The various polymer solutions were each printed or deposited onto acarbon fibre enforced epoxy prepreg, designated as Cycom977-2 (suppliedby Cytec Engineered Materials of Wrexham, UK) as a substrate.

In each case the as-printed substrates were cured according to thefollowing cure regime:

-   -   {circle around (1)} Ramp: 20° C. →100° C. (rate: 2° C./min)    -   {circle around (2)} Dwell: 30 min    -   {circle around (3)} Ramp: 100° C. →177° C. (rate: 2° C./min)    -   {circle around (4)} Dwell: 120 min    -   {circle around (5)} Ramp: 177° C.→20° C. (rate: 2° C./min)

In order to investigate the behaviour of the islands of thermoplasticsdue to exposure to elevated temperatures of the curing cycle a series ofmicroscopy studies was undertaken:

EXAMPLE 1A

Referring to FIG. 1, a series of PMMA islands were applied to a glassslide. The PMMA was doped with fluorescein to enhance optical microscopyand images were taken with an optical microscope (CK40-SLP, Olympus).

FIG. 1a shows islands prior to heating and FIG. 1b shows the sameislands after exposure to the curing cycle. FIGS. 1a ′ and 1 b′ areimages of the same solutions deposited in a hexagonal array. FIG. 1cshows the spots after being exposed to an elevated temperature for 2hours.

The results show that the islands reduce in volume (due to solventevaporation), and demonstrate some ‘coffee staining’ as a result ofevaporation of the solvent, but they remain in place and are not subjectto translational motion.

EXAMPLE 1B

Referring to FIG. 2 a series of PMMA islands were applied to a glassslide. The PMMA was doped with fluorescein to enhance optical microscopyand fluorescein images were taken with an optical microscope (ImageXpress).

FIG. 2a is before curing, FIG. 2b is after curing and FIG. 2c is afterbeing heated to 100° C. for 2 hours.

The results clearly show that the dots stay as distinct islands.

EXAMPLE 1C

Referring to FIG. 3 a series of PMMA dots were applied to Cycom977-2.Images were taken with an optical microscope (Polyvar).

FIG. 3a is before curing, FIG. 3b is after curing and FIG. 3c is afterbeing heated to 100° C. for 2 hours.

In this case there is no visible evidence of the thermoplastic islandson the surface after exposure to heat, however, we believe that thethermoplastic remains, as evidenced in the previous images.

EXAMPLE 1D

Referring to FIG. 4 a series of fluorescein-doped PMMA dots were appliedto Cycom977-2. Fluoroscein images were taken with an optical microscope(Image Xpress).

FIG. 4a is before curing, FIG. 4b is after curing and FIG. 4c is afterbeing heated to 100° C. for 2 hours.

In this case it appears that the fluorescein within the islands sprayedout after heating and with increased temperature (there is some evidenceof fluorescein in FIG. 4c ).

In order to investigate this behaviour a series of interferometryexperiments were undertaken.

EXAMPLE 1E

Using a 3D optical microscope (Contour GT supplied by Bruker)interferometry images were taken of PMMA dots applied to a glass slide,before curing (FIG. 5a ) and after curing (FIG. 5b ).

The images show that the islands remain in place.

EXAMPLE 1F

Using a 3D optical microscope (Contour GT supplied by Bruker)interferometry images were taken of PMMA dots applied to Cycom977-2,before curing (FIG. 6a ) and after curing (FIG. 6b ).

The results show that islands are evident before curing but not aftercuring.

Whilst not wishing or intending to be bound by any, particular theory,we believe that the islands have either splayed out or have penetratedinto the surface of the prepreg. However, we believe that there arelocal peak concentrations of thermoplastics islands at or towards thesurface.

EXAMPLE 2A

Referring to FIG. 7 a series of fluorescein-doped PEG dots were appliedto Cycom977-2. Fiuoroscein images were taken with an optical microscope(Image Xpress).

FIG. 7a is before curing, FIG. 7b is after curing and FIG. 7c is afterbeing heated to 100° C. for 2 hours.

In this case it appears that the fluorescein within the islands sprayedout after heating and with increased temperature (there is some evidenceof fluorescein in FIG. 7c ).

A series of further experiments were undertaken to determine thephysical performance of the composites made according to the invention.

EXAMPLES 3 TO 6 Interlaminar Shear Strength

In each case a composite test piece was made either using eight prepreglayers (designated virgin), or with eight prepreg layers and thesolutions set out in Table 1.

Determination of apparent interlaminar shear strength by short-beammethod (SBS) according to BS EN ISO 14130:1998.

-   -   Calculation of interlaminar shear stress: τ=3F/4bh &        τ_(M)=3F_(M)/4bh    -   Where: F is the load, F_(M) is the maximum load;    -   b is the width of the test specimen; b=10.0 mm    -   h is the thickness of the test specimen; h=2.0 mm

EXAMPLE 3

The results for repeat interlaminar sheer strength tests (5 runs each)are shown in FIG. 8 for a series of experiments conducted on test pieceswhich are as cured.

The results are as shown in Table 3:

TABLE 3 Results Slope Maximum Load τ_(M) (×10³ N/mm) (×10³ N) (MPa) Avg.SD Avg. SD Avg. SD Virgin 5.340 0.355 2.985 0.228 111.9 8.5 5% PEG +Ethanol 5.781 0.177 2.817 0.107 105.6 4.3 5% PEG + Water 7.093 0.2393.590 0.105 134.6 3.9 5% PMMA + DMF 7.147 0.082 3.118 0.081 116.9 3.0*SD: standard deviation

It can be seen that samples with printed self-healing agent have higherstiffness which is represented by slope of straight part of load versusextension curves than that of virgin samples. And with printed PMMA,samples have the highest stiffness. Moreover, there is no significantdifference among virgin and self-healing agent printed samples regardingto average maximum load and average maximum interlaminar shear stress,showing no reduction in the structural integrity of the system due tothe deposited self-healing agent. That said, there is a notablereduction in the standard deviation for 5% PMMA printed system. Althoughwe neither wish nor intend to be bound by any particular theory, webelieve this to be due to a better damage control by arresting crackpropagation through PMMA islands, whilst maintaining the adhesionbetween the prepreg plies, and hence an increased engineeringpredictability in the optimised system.

EXAMPLE 4

The results for repeat interlaminar sheer strength tests (5 runs each)are shown in FIG. 9 for a series of experiments conducted on test pieceswhich are as cured and subsequently ‘healed’ by exposing to the cureregime as set out above.

The results are as shown in Table 4:

TABLE 4 Results Slope Maximum Load τ_(M) (×10³ N/mm) (×10³ N) (MPa) Avg.SD Avg. SD Avg. SD Virgin 5.900 0.368 3.170 0.269 118.9 10.1 5% PEG +Ethanol 6.780 0.044 3.088 0.128 115.8 4.8 5% PEG + Water 6.906 0.1453.363 0.088 126.1 3.3 5% PMMA + DMF 7.205 0.406 3.177 0.066 119.1 2.5

It can be seen that samples with printed self-healing agent have higherstiffness than that of virgin samples. And with printed PMMA, sampleshave highest stiffness. Moreover, there is no significant differencebetween virgin samples and self-healing agent printed samples regardingto average maximum load and average maximum interlaminar shear stressvalues, showing no reduction in the structural integrity of the systemdue to the deposited self-healing agent.

Comparing the results from Examples 3 and 4, it can be seen that theaverage maximum interlaminar shear stress (τ_(M)), average maximum loadand average stiffness of almost four groups are slightly enhanced afterthe healing cycle, it could be either caused by the printed self-healingagent or by the post curing of epoxy in pre-preg itself or both.

EXAMPLE 5

The results for repeat interlaminar sheer strength tests (5 runs each)are shown in FIG. 10 for a series of experiments conducted on testpieces which are as cured and subsequently ‘damaged’ by exposing thetest piece to a tensometer and stopping loading just after the maximumload displaced is seen on the monitor.

The results are as shown in Table 5:

TABLE 5 Results Slope Maximum Load τ_(M) (×10³ N/mm) (×10³ N) (MPa) Avg.SD Avg. SD Avg. SD Virgin 4.268 0.172 2.463 0.079 92.4 3.6 5% PEG +Ethanol 5.620 0.283 2.376 0.235 89.1 8.8 5% PEG + Water 6.061 0.5373.054 0.318 114.6 11.9 5% PMMA + DMF 5.770 0.393 2.593 0.073 97.3 2.7

We can see (by comparing the results from Example 3 and those in Table5) that the mechanical properties of four groups are reduced afterdamage, as expected. Samples with printed self-healing agent arestiffener than the virgin ones. Moreover, no significant difference isobserved regarding to average maximum load and average maximuminterlamiriar shear stress among four groups, showing no reduction inthe structural integrity of the system due to the deposited self-healingagent. Since the damage process cannot guarantee introducing the sameamount of damage to every test specimen, the results of some groups havea variation which does not obey the common rules.

EXAMPLE 6

The results for repeat interlaminar sheer strength tests (5 runs each)are shown in FIG. 11 for a series of experiments conducted on testpieces which are as cured and subsequently ‘damaged’ by exposing thetest piece to a tensometer and stopping loading just after the maximumload displaced is seen on the monitor and subsequently ‘healed’ byexposing to the cure regime as set out above.

The results are set out in Table 6.

TABLE 6 Results Slope Maximum Load τ_(M) (×10³ N/mm) (×10³ N) (MPa) Avg.SD Avg. SD Avg. SD Virgin 5.046 1.011 2.520 0.406 94.5 15.2 5% PEG +Ethanol 5.224 0.464 2.486 0.300 93.2 11.3 5% PEG + Water 6.334 0.5022.881 0.339 108.1 12.7 5% PMMA + DMF 5.756 0.316 2.674 0.174 100.3 6.5

It can be seen that samples with printed self-healing agent are slightlystiffener than virgins. And comparing the results of Table 6 with thoseof Table 5, almost every parameter of each group has a slightenhancement, except τ_(M) of 5% PEG+Water group which, we believe maywell be a variation of experiment process. Because the damage processcannot guarantee all damaged samples were introduced the same amount ofdamage, that means the results of Table 6 samples in have higher damagelevel than the samples set out in Table 5, the values of all parametersof the Example 6 should lower than that of Example 5, in which case, wewill not be able to see the self-healing efficiency if the self-healingefficiency is not apparent enough.

The summary of the results is set out in Table 7 below:

TABLE 7 Results Summary of SBS tests Average value n = 5 SlopeLoad_(MAX) τ_(M) Damage Heal (×10³ N/mm) SD* (×10³ N) SD* (MPa) SD*Virgin x x 5.340 0.355 2.985 0.228 111.9 8.5 x ✓ 5.900 0.368 3.170 0.269118.9 10.1 ✓ x 4.268 0.172 2.463 0.079 92.4 3.0 ✓ ✓ 5.046 1.011 2.5200.406 94.5 15.2 5% PEG + x x 7.093 0.239 3.590 0.105 134.6 3.9 Water x ✓6.906 0.145 3.363 0.088 126.1 3.3 ✓ x 6.061 0.537 3.054 0.318 114.6 11.9✓ ✓ 6.334 0.502 2.891 0.339 108.1 12.7 5% PEG + x x 5.781 0.177 2.8170.107 105.6 4.3 Ethanol x ✓ 6.780 0.044 3.088 0.127 115.8 4.8 ✓ x 5.6200.283 2.376 0.235 89.1 8.8 ✓ ✓ 5.224 0.464 2.486 0.300 93.2 11.3 5%PMMA + x x 7.147 0.082 3.118 0.081 116.9 3.0 DMF x ✓ 7.205 0.406 3.1770.066 119.1 2.5 ✓ x 5.770 0.393 2.593 0.073 97.3 2.7 ✓ ✓ 5.756 0.3162.674 0.174 100.3 6.5

From SBS test, it can be concluded as following:

-   -   a. Printed polymeric agent of only 0.02% weight addition can        significantly increase the shear stiffness of the composite        before and after damage in shear, where the composite is most        vulnerable and such improvements can significantly improve the        damage tolerance of the overall system.    -   b. Even though this experiment was conducted to evaluate        structural soundness of the system alone, there is evidence of        self-healing efficiency as a result of the damage process,        showing no reduction in the structural integrity of the system        due to the deposited self-healing agent.

EXAMPLES 7-10

A further series of experiments were undertaken to determine the mode Iinterlaminar facture toughness, G_(IC) for unidirectionally reinforcedmaterials, according to BS ISO 15024:2001. Each test sample, whether a‘virgin’ or test sample, comprised twelve layers of prepreg material,the difference between ‘virgin’ and ‘test’ samples being the provisionof thermoplastic polymer at the interface between successive layers inthe ‘test’ samples.

According to the standard, there are several important G_(Ic) values ofparticular points as follows:

-   1) NL point—non linear point of deviation from linearity on the load    versus extension trace-   2) 5%/MAX point—the point which occurs first on the loading the    specimen between:    -   a) The point of 5% increase in compliance (C₅%) from its initial        value (C₀);    -   b) The maximum load point.-   3) PROP points—propagation value of fracture toughness, consisting    of points of discrete delamination length increments beyond the tip    of the insert or starter crack tip marked on the load-extension    trace, points where the crack has been arrested being excluded

The important points are shown graphically below:

COMPARATIVE EXAMPLE 7

In order to determine a base line, ‘virgin’ samples were tested. Thetested samples were then subjected to the ‘heal’ procedure and testedagain to determine if any self-healing occurred.

The results are shown in FIG. 12 and set out below in Table 8:

TABLE 8 Results for Comparative Example 7 G_(IC) (kJ/m²) G_(IC) (kJ/m²)Before healing cycle After healing cycle Avg. Avg. NL 5%/MAX PROP NL5%/MAX PROP point point points point point points 1 0.12473 0.147230.1825 0.03625 0.05311 0.07353 2 0.15226 0.17972 0.17082 0.06807 0.081220.07775 3 0.12558 0.12749 0.12761 0.06182 0.06327 0.062 4 0.150130.16851 0.16379 0.07914 0.08282 0.07273 5 0.11611 0.12309 0.12177 0.08730.09164 0.08688 Average 0.13376 0.14921 0.1533 0.06652 0.07441 0.07458SD 0.01636 0.02481 0.02704 0.01957 0.01575 0.009

EXAMPLE 8

An experiment was conducted using a composite formed from Cycom977-2 andprovided with dots applied in accordance with Example 2 (as set out inTable 1 and 2), i.e. 5% PEG in water. The samples were tested. Thetested samples were then subjected to the ‘heal’ procedure and testedagain to determine if any healing occurred.

The results are shown in FIG. 13 and set out in Table 9

TABLE 9 Results of Example 8 G_(IC) (kJ/m²) G_(IC) (kJ/m²) Beforehealing cycle After healing cycle Avg. Avg. NL 5%/MAX PROP NL 5%/MAXPROP point point points point point points 1 0.21025 0.22645 0.25724 — —— 2 0.18471 0.19142 0.22501 0.1845 0.19109 0.19153 3 0.17013 0.193760.21784 0.13057 0.1328 0.13037 4 0.1546 0.17097 0.20639 0.11506 0.119480.12052 5 0.19575 0.2283 0.234 0.1435 0.18661 0.18015 Average 0.182260.20218 0.22798 0.14341 0.1575 0.15564 SD* 0.02307 0.02466 0.019130.02976 0.03666 0.03541

EXAMPLE 9

An experiment was conducted using a composite formed from Cycom977-2 andprovided with dots applied in accordance with Example 3 (as set out inTable 1 and 2). i.e. 5% PEG in ethanol.

The samples were tested. The tested samples were then subjected to the‘heal’ procedure and tested again to determine if any healing occurred.

The results are shown in FIG. 14 and set out in Table 10

TABLE 10 Results of Example 9 G_(IC) (kJ/m²) G_(IC) (kJ/m²) Beforehealing cycle After healing cycle Avg. Avg. NL 5%/MAX PROP NL 5%/MAXPROP point point points point point points 1 0.14259 0.15983 0.177210.11871 0.12171 0.11021 2 0.18581 0.18884 0.19879 0.06709 0.069280.05951 3 0.11545 0.13142 0.13464 0.09012 0.09193 0.08407 4 0.120660.13389 0.13222 0.08993 0.09331 0.08488 5 0.16128 0.17188 0.172790.11456 0.11487 0.09212 Average 0.14516 0.15717 0.16313 0.09608 0.098220.08616 0.02916 0.02465 0.02885 0.02102 0.0208 0.01824

EXAMPLE 10

An experiment was conducted using a composite formed from Cycom977-2 andprovided with dots applied in accordance with Example 1 (as set out inTable 1 and 2), i.e. 5% PMMA in DMF.

The samples were tested. The tested samples were then subjected to the‘heal’ procedure and tested again to determine if any healing occurred.

The results are shown in FIG. 15 and set out in Table 11

TABLE 11 Results for Example 10 G_(IC) (kJ/m²) G_(IC) (kJ/m²) Beforehealing cycle After healing cycle Avg. Avg. NL 5%/MAX PROP NL 5%/MAXPROP point point points point point points 1 0.32722 0.32753 0.243470.20649 0.21411 0.15388 2 0.23818 0.28664 0.25612 0.11919 0.121320.11574 3 0.36688 0.37538 0.27223 0.19209 0.19501 0.17671 4 0.258260.26611 0.19555 0.13601 0.15049 0.13204 5 0.25664 0.25858 0.18249 0.15420.1555 0.11857 Average 0.28944 0.30285 0.22997 0.1616 0.16729 0.13939SD* 0.055 0.04859 0.03902 0.03692 0.03706 0.02574

Referring now to FIG. 16, we can see the average G_(Ic) values of NL,5%/MAX and avg. PROP points of PMMA printed specimens are higher thanvirgin and PEG printed specimens both before and after healing cycles,which means the printed PMMA enhances the interlaminar fracturetoughness of all specimens. Whilst the PEG+ethanol results are onlymarginally better than the virgin results there is a slight improvementfor this system as well.

Advantageously, the use of ink jet printing allows for easy applicationof the thermoplastic materials. Moreover, using direct applicationtechniques (or indeed removal techniques) it is possible to introducefunctionally graded improvements in fracture toughness, i.e. to have aresistance to fracture which changes across the width, or length of apart. FIG. 17 provides a schematic of two proposed parts with differentregions bearing the thermoplastic polymer.

EXAMPLE 11

The results, shown in FIG. 18, show results for crack resistance in apart in which regions that were partly virgin and partly printed withpolymer patterns. The graph shows how G_(IC) swaps over and increasesonce the fracture reaches the patterned region.

EXAMPLE 12

In FIG. 19, G_(IC) is seen to improve as the printed polymerconcentration is increased (in each dot). The fracture toughness of thesystem printed with 20% polymer concentration (upper line) issignificantly higher compared to 10% system.

EXAMPLE 13

In a further set of experiments the stiffness of samples (before andafter heating) were determined. The results are shown in FIGS. 20 and21. The samples were virgin (v), as Example 1 (a), as Example 2 (b) andas Example 3 (c).

It was demonstrated that samples containing the printed thermoplasticpolymer have up to a 33.8% increase in stiffness compared to the virginsamples.

EXAMPLE 14

In a further set of experiments Cycom977-2 was printed with higherconcentration of polymer (20% PMMA in DMF) in dots applied in accordancewith Example 10.

The samples were tested. The tested samples were then subjected to the‘heal’ procedure at slightly lower temperature of 160° C. for 40 minutesto avoid the reduction in toughness seen in the previous data due to thesecondary cross-linking that occurs at 177° C., and tested again todetermine if any healing occurred.

The results are shown in FIG. 22. The graphs show non-printed controlsamples (NP) and samples printed with an array of dots comprising 20wt/wt % of PMMA. The fracture toughness was significantly increased bothbefore and after self-healing treatment (i.e. heating to 160° C. for 40mins), and the standard deviation was significantly reduced for thesystem with 20% concentration PMMA printed patterns.

EXAMPLE 15

Synchrotron X-ray computed tomography was carried out on the samples atJoint Engineering, Environmental and Processing (JEEP) beam line atDiamond Light Source. 1800 projections were recorded with the exposuretime of 0.1 second over a 180-degree rotation; the beam energy was 53keV. The distance between the sample and the detector was shorttherefore no phase contrast was observable. The detector was 2560×1373pixels with a resolution of 3.24 μm per voxel. The width of thespecimens were bigger than the field of view and region of interesttomography was performed which due to low attenuation of carbon did notrequire a ROI correction during reconstruction. A back projection codewith limited ring-artifact suppression was used to reconstruct the data.

The evidence of self-healing is presented in FIG. 23, showing the SBStested samples before (left) and after (right) thermal treatment withthe layers being fused in the self-healed sample.

EXAMPLE 16

A fracture toughness test was carried out to evaluate the differencebetween interleaf (film) area between the plies, commonly used toincrease the toughness in composites at the expense of other mechanicalproperties due to the loss of adhesion between the plies, and the herepresented printed patterns which allow epoxy surfaces in CFRP to remainin contact. The systems are presented in FIG. 24 (a-c), as a) 20%concentration film between the plies, b) 20% printed dots in hexagonalpatterns (equivalent to 10% film deposited volume fraction of PMMA) andc) summary of the results for three systems, showing a notable increasein fracture toughness for 20% PMMA printed dots in CFRP compared to 10%printed full area or film in the same system.

As the amount (%) of PMMA is increased in film printed samples, thestandard deviation is also increased, whereas the standard deviation isdecreased for 20% PMMA dots due to a better crack arrest and higherdegree of engineering predictability. 20% film pattern is only shown forthe standard deviation purposes, considering that it would require closeto 50% concentration of PMMA in hexagonal patterns to provide acomparative result for the discrete system, which is difficult toachieve using the existing inject printing system. The 20% filmcomposite would also significantly increase the weight.

These results demonstrated the improved G_(IC) for patterned surfacesand reduced standard deviation of the system at only 0.02% addition ofdiscrete PMMA islands, compared to the film printed ‘interleaf’ methodin the same system, if a comparable volume fraction of PMMA is used.

IN CONCLUSION

From the optical, fluorescein and interferometry images of PMMA (andother) dots on glass slides we can see the printed PMMA dots can stay asdroplets after curing cycle. But from fluorescein and interferometryimages of PMMA dots on pre-preg, we cannot see any dots after curingcycle, which means the printed PMMA is likely to react with epoxy andform localised bonds whilst fluorescein may depart due to the hightemperature.

In the experiments set out above, in order to investigate theself-healing efficiency, a damage process has been employed to introducean appropriate amount of damage into specimens for self-healing agencyto heal. Double cantilever beam (DCB) and short beam shear (SBS) testshave been adopted to evaluate the self-healing efficiency, due to thedamage occurring between the composite plies during the tests.Fluorescein was added to ink in order to investigate behaviours of theprinted polymer dots before and after heating to different temperatures.From optical, fluorescein and interferometry images of PMMA dots onglass slides, it can be seen the printed PMMA dots stay as dropletsafter curing cycle. The fluorescein sprayed out after curing cycle bothin PEG and PMMA cases, which can be spotted from fluorescein images ofpolymer dots on pre-preg. From interferometry images of PMMA dots onpre-preg, no dots are visible after curing cycle.

From the SBS test results, it is apparent to see that the printedself-healing agent can stiffen composite materials both before and afterself-healing, and that printed PMMA samples have the highest stiffnessamong the four groups. Besides, we can see the values of slope, maximumload and τ_(M) of samples after healing are higher slightly than that ofbefore healing both in undamaged groups and damaged groups. This may beeither because of the post curing of the epoxy in pre-preg or theprinted self-healing agent but we prefer the latter explanation. Sincethe damage method cannot guarantee introducing the same amount ofdamages into samples, some results did not obey the common rules, whichcaused some variations.

From DCB test results, we can see almost all average values of initial(non-linear), 5% of the maximum load and average propagation values offracture toughness with printed PMMA are higher than that of virgin andPEG printed specimens both before and after healing cycle, whichindicated that printing PMMA between composite plies significantlyenhanced the interlaminar fracture toughness, both before and after thethermally treated damage (self-healing process).

It is clear that printed self-healing agent can stiffen the composite,which can be concluded from SBS test results. Samples with printed PMMAhave the highest stiffness among the four groups. However, no apparentself-healing efficiency has been observed among virgin samples andsamples with self-healing agent through SBS test. It can be seen thatspecimens with printed PMMA have the highest mode I interlaminarfracture toughness (G_(IC)) both before and after healing cycles amongthe four groups, indicating a substantial recovery of the material afterintroduced damage, and a significant increase in fracture toughness ofthe system before the damage has been introduced. This also implies thatthe material is likely to sustain more load, due to its improvedcapacity to resist crack initiation and propagation. Hence, the overallservice life of the material will improve and also impart self-healingproperty, further extending its durability and lowering maintenancecosts. This simultaneous increase in a number of thermo-mechanicalproperties both before and after damage, is achieved at less than 0.1%weight increase, rendering this as a unique system, capable of impartingproperties into the original material that could not be achieved in anyother way.

In respect of improving the self-healing properties, we believe,although we do not wish or intend to be bound by any particular theory,that increasing the molecular weight of the polymer will lead toimproved self-healing. We also believe, and our experiments bear out,that increasing the amount of polymer will lead to increased robustnessof the dots during cure and will lead to markedly improved self-healingproperties. This, we believe, is due to increased viscosity of thepolymer in the solution and/or islands of higher concentration.

1. A method of fabricating a composite material, the method comprisingthe steps of a) providing a first layer of a fibre reinforced polymer,preferably a thermoset FRP, b) providing an array of thermoplasticislands across at least a proportion of a major surface of the firstlayer, c) providing a second layer of a fibre reinforced polymer,preferably a thermoset FRP, d) laying the second layer over at leastsome of the islands, and e) securing the first and second layerstogether.
 2. A method according to claim 1, comprising securing thefirst and second layers together by the application of heat and/orpressure.
 3. A method according to claim 1 or 2, comprising providing aseach of the first and second layers a prepreg, for example a carbonfibre reinforced prepreg, preferably an epoxy matrix.
 4. A methodaccording to any of claim 1, 2 or 3, wherein the islands are provided soas to extend over 20-80% of the area of the first layer, ideally 40-60%.5. A method according to any of claims 1 to 4, comprising applying thethermoplastic islands across at least a proportion of a major surface ofthe first layer as a solution of thermoplastic in a solvent.
 6. A methodaccording to any preceding claim, comprising providing the islands bymeans of one of ink jet printing, contact printing and non-contactprinting or by use of a mask and spray method, preferably ink jetprinting.
 7. A method according to any preceding claim, comprisingrepeating steps b) to e) to form successive layers of the compositematerial.
 8. A composite material comprising a first and second layerand a thermoplastic material, the first and second layer each comprise athermoset fibre reinforced polymer, the thermoplastic material isdiscontinuously distributed at and/or towards the interface between thefirst and second layer.
 9. A material according to any of claims 8,wherein the first and second layer are formed from the same or differentthermoset polymers and/or may comprise the same or different fibres. 10.A material according to claim 8 or 9, wherein the thermoset polymers areselected from epoxy, polyesters, vinyl esters, phenolic resins and anyother cross-linked thermosetting polymer.
 11. A material according toany of claim 8, 9 or 10, wherein the fibres are continuous and/ordiscontinuous and/or may be provided as aligned filaments or tows, or asa braided, knitted or woven mat or sheet.
 12. A material according toany of claims 8 to 11, wherein the fibres are selected from one or moreof carbon fibres, aramid fibres, glass and glass ceramic fibres,plastics fibres (e.g. Nylon, Terylene), hemp, wood and/or other organicfibres, inorganic fibres.
 13. A material according to any of claims 8 to12, wherein the volume fraction of fibre-to-polymer is from 0.01 to 5%,most preferably from 0.05 to 1% within the first and/or second layer.14. A material according to any of claims 8 to 13, wherein thediscontinuous distribution extends over some or all of the interface,and/or may be localised in certain defined regions.
 15. A materialaccording to any of claims 8 to 14, wherein the discontinuousdistribution is in the form of an array of islands or zones, for exampleislands or zones of peak local concentration.
 16. A material accordingto claim 15, wherein the islands comprise one or more thermoplasticpolymers.
 17. A material according to claim 15 or 16, wherein theislands comprise a first array of islands and a second array of islands,each of the first and second arrays being formed of differentthermoplastics materials.
 18. A material according to any of claim 15,16 or 17, wherein the islands extend over 20-80% of the area of thefirst layer, preferably 40-60%.
 19. A material according to any ofclaims 15 to 18, wherein each of the islands has an area of less than 1mm², preferably less than 0.5 mm², for example, less than 0.4, 0.3, 0.2,0.1 mm².
 20. A material according to any of claims 8 to 19, wherein thethermoplastic material or polymer has a melting point of 50 to 200° C.,preferably 75 to 190° C., most preferably 100 to 180° C.
 21. A materialaccording to any of claims 8 to 20, wherein the thermoplastic materialor polymer has a molecular weight (NAMW) in the range of 3000 to 50000,preferably 4000 to
 40000. 22. A material according to any of claims 8 to21, wherein the thermoplastic material or polymer is selected frompoly(ethylene glycol) (PEG), poly(methyl methacrylate) (PMMA) andsimilar thermoplastic polymers, preferably poly(ethylene glycol) andpoly(methyl methacrylate).
 23. A material according to any of claims 8to 23, comprising a further intervening layer comprising an array ofislands of thermoplastic polymer provided on the second layer.
 24. Amaterial according to claim 23, comprising a third layer comprising afibre reinforced polymer, preferably a thermoset FRP, overlying thefurther intervening layer.
 25. A composite, preferably a self-healingcomposite, the composite comprising a first layer of fibre reinforcedthermoset polymer and a second layer of fibre reinforced thermosetpolymer and a thermoplastic polymer distributed at and/or towards theinterface there between, the composite being stiffer than an equivalentfibre reinforced thermoset polymer composite absent the thermoplasticpolymer.
 26. A composite, preferably a self-healing composite, thecomposite comprising a first layer of fibre reinforced thermoset polymerand a second layer of fibre reinforced thermoset polymer and athermoplastic polymer distributed at and/or towards the interfacetherebetween, the composite having improved stiffness and/orinterlaminar shear strength.
 27. A composite according to claim 25 or26, wherein the thermoplastic polymer is discontinuously distributed atand/or towards the interface between the first and second layer.
 28. Acomposite according to claim 25, 26 or 27, wherein the thermoplasticpolymer is preferentially distributed at or towards the interface suchthat the relative concentration of thermoplastic polymer is at a peak ator towards the interface.